BAGHDAD (AP) — A new wave of car bombs rocked commercial streets in the Iraqi capital on Tuesday, killing 20 in the latest apparent attack by hard-line Sunni insurgents aiming to undermine confidence in the Shiite-led government.

Meanwhile, Sunni leaders in Basra said unknown gunmen had shot dead 17 Sunnis in the Shiite-dominated city over the past two weeks, following threats to retaliate against them for attacks on Shiites in other parts of Iraq.

Sunnis in Basra were frequently targeted during the widespread 2004-2008 sectarian killings that pushed the country to the brink of civil war. The shootings in the southern port city are likely to raise fears that Iraq may be drifting back toward the cycle of violence in those years that left thousands dead every month.

Abdul-Karim al-Khazrachi, who leads the Sunni Endowment that oversees holy sites in the city, said in a statement issued late Monday that the sect had decided to close down its mosques due to “grave security deterioration and the continuation of the sectarian killings.”

Khazrachi told The Associated Press in a phone interview from Baghdad that the killings were preceded by threats — letters that came with bullets in the envelopes, and text messages — that vowed revenge for insurgent attacks against Shiites across Iraq. The letters demanded that Sunnis leave the province. He said he didn’t know the killers’ identities.

The slain, he added, included clerics, worshippers and others. The latest was a 70-year old grocer who was shot dead by gunmen while standing in his store Monday night. A police officer in the city confirmed the 17 killings, speaking on condition of anonymity as he was not authorized to talk to media.

Khazrachi said the mosque closure was intended to “protect the Sunnis in the province” but was “also a message to all those in charge of the security to shoulder their responsibilities.”

Basra is Iraq’s second-largest city and is located about 550 kilometers (340 miles) southeast of Baghdad. It was controlled by militias for years before the U.S.-backed Iraqi army gained control following a series of offensives in 2008.

In Baghdad, meanwhile, a wave of car bombs hit commercial districts.

The deadliest blast hit the northeastern suburb of Husseiniyah, killing five people and wounding 14 others.

Shortly before sunset, another bomb went off near a juice shop in central Baghdad, killing three and wounding 21.

Two more died in another car bomb explosion near a restaurant downtown, and in western Baghdad, two blasts killed six people and wounded 20 others.

A final two blasts hit a commercial street in the capital’s southeast, killing four and wounding 71.

In the western city of Fallujah, security forces foiled an assault on a police station, killing four attackers. Two policemen were killed, officials said.

Medical officials confirmed the casualty figures. All spoke anonymously as they weren’t authorized to release information.

Insurgents are believed to be trying to capitalize on Sunni discontent against what they consider to be second-class treatment by the Shiite-led government.

Violence surged in April after government troops moved against a camp of Sunni demonstrators in the town of Hawija, about 240 kilometers (150 miles) north of Baghdad, triggering clashes that killed 44 civilians and a member of the security forces, according to a U.N. estimate.

Since then, bomb attacks claimed by or blamed on al-Qaida’s local branch have killed hundreds of Shiite civilians in mosques, markets and elsewhere. Apparent retaliatory attacks on Sunni mosques have been less bloody overall, but have still claimed dozens of lives.

More than 4,000 people have been killed, including 804 just in August, according to United Nations figures. The monthly death tolls are the highest since 2008.

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Associated Press writers Sinan Salaheddin in Baghdad and Nabil al-Jurani in Basra contributed to this report.